Overbore

Overbore cartridges are those with a relatively large case volume or case capacity, coupled with a relatively small diameter bullet.

The case volume or case capacity and barrel bore area can be mathematically related to obtain a case volume to bore area ratio in metric or imperial units.

${\displaystyle {O_{ratio}}={\frac {Volume_{case}}{Area_{bore}}}}$

where:

• ${\displaystyle Volume_{case}}$ = the cartridge case internal volume or case capacity (in ml or (for non-metric users) grains of water)
• ${\displaystyle Area_{bore}}$ = barrel bore cross section area (in cm² or in²)

The higher the O_ratio result, the more overbore a cartridge will be. As the ratio is expressed in units of length, relatively high O_ratio is a good predictor of suitability for relatively long barreled guns.

O_ratio is also used to predict barrel life in cartridges of the same caliber, but not of different calibres, since the ratio is an extensive quantity that does not correlate to temperature or pressure (e.g. a .50 cal straight cartridge may have the same overbore as a highly necked down .17 cal cartridge).